Power series predistortion is a method for compensating for non-linear distortion of microwave transmission power amplifiers (S. Mizuta, Y. Suzuki, S. Narahashi, and Y. Yamao, “A New Adjustment Method for the Frequency-Dependent IMD Compensator of the Digital Predistortion Linearizer,” IEEE Radio and Wireless Symposium 2006, pp. 255-258, January 2006).
A predistorter adds a distraction compensation signal to an input transmission signal to compensate for distortion components generated in a power amplifier due to non-linearity of the power amplifier. FIG. 1 illustrates an exemplary configuration of a conventional power series digital predistorter (hereinafter simply referred to as digital predistorter) 100P. In the example, a digital input transmission signal includes an I-phase signal and a Q-phase signal (also called I/Q signals). Accordingly, the same components are provided in pairs as required, one in the path for the I signal provided to an input terminal 7I and the other in the path for the Q signal provided to an input terminal 7Q. Since the I/Q signal paths are well known in the art, individual description of the I/Q signal paths will be omitted and input I/Q signals are also simply referred to as an input transmission signal in the following description.
The digital predistorter 100P includes a divider 11, a linear transmission path 12 including a delay circuit, a third-order distortion generating path PDG3 including a third-order distortion generator 131 and a third-order distortion vector regulator 141, a combiner 15 which combines an output from the linear transmission path 12 with an output from the third-order distortion generating path PDG3, an amplifier 60, a directional coupler 21 which extracts a portion of an output from the amplifier 60 as a feedback signal, a frequency down-converter 22 which converts the frequency of the feedback signal, a quadrature demodulator 23 which quadrature-demodulates the downconverted feedback signal, an analog-to-digital converter 31 which converts the demodulated I/Q feedback signals to digital signals, and a third-order distortion vector controller 321 which detects a distortion comportment from the digital feedback signals converted from the analog feedback signals and adjusts vector coefficients (amplitude and phase) to be set in the third-order distortion vector regulator 141. The directional coupler 21, the frequency down-converter 22, and the quadrature demodulator 23 make up a feedback signal generating part 20P.
The amplifier 60 includes a digital-to-analog converter 61 which converts input digital I/Q signals to which a compensation signal is added to analog I/Q signals, a quadrature modulator 62 which quadrature-modulates the analog I/Q signals, a frequency up-converter 63 which converts the frequency of the modulated outputs to a carrier frequency, and a power amplifier 64 which amplifies the power of up-converted signal. The power-amplified up-converted signal is provided through an output terminal 8 to, for example, an antenna through a duplexer, not shown.
A digital transmission signal input into the digital predistorter 100P is divided and distributed by the divider 11 to the linear transmission path 12 and the third-order distortion generating path PDG3. In the third-order distortion generating path PDG3, the distributed input transmission signal is raised to the third power by the third-order distortion generator 131 to generate a third-order distortion component. The phase and amplitude of the third-order distortion component are adjusted by using vector coefficients (phase and amplitude) set in the third-order distortion vector adjustor 141 to obtain a compensation signal (the adjustment of the phase and amplitude of a distortion component to obtain a compensation signal is hereinafter referred to as vector adjustment). The time delay of the input transmission signal traveling from the divider 11 to the combiner 15 through the linear transmission path 12 is adjusted by a delay circuit, not shown, included in the linear transmission path 12 so that the time delay becomes equal to the time delay of the input transmission signal traveling from the divider 11 to the combiner 15 through the third-order distortion generating path PDG3, that is, the timings of the arrivals of signals from both paths become the same. The input transmission signal from the linear transmission path 12 and the compensation signal from the third-order distortion generating path PDG3 are added together by the combiner 15 and provided to'the amplifier 60 as an output of the predistorter 100P.
A portion of the output signal from the power amplifier 64 is extracted by the directional coupler 21 as a feedback signal, which is then converted to a frequency in an intermediate frequency band by a frequency down-converter 22 and demodulated by the quadrature demodulator 23 to I/Q signals. The demodulated I/Q signals are converted to digital feedback signals by the analog-to-digital converter (ADC) 31 and provided to the third-order distortion vector controller 321. The third-order distortion vector controller 321 observes third-order distortion components in adjacent bands to a main signal component band in the digital feedback signals and controls the vector coefficients to be set in the third-order distortion vector regulator 141 to minimize the power of the third-order distortion component, thereby compensating for the third-order distortion component which have been generated in the power amplifier 64.
In FIG. 1, feedback signals input into the ADC 31 contain a main signal component (where the main signal component corresponds to the input transmission signal input into the digital predistorter) as well as distortion components generated in the power amplifier 64. If analog-to-digital conversion at the ADC 31 is performed by setting the full scale of the ADC 31 to a value determined in view of an expected maximum value of the input transmission signal level, a sufficient accuracy of quantization of a distortion component to be detected cannot be achieved since the power of the distortion component is lower than that of the main signal component. Consequently, a change in the third-order distortion component of the output from the power amplifier 64 cannot adequately be observed in terms of magnitude at the third-order distortion vector controller 321 even though the vector coefficients to be set in the third-order distortion vector regulator 141 is controlled by the third-order distortion vector controller 321. The third-order distortion vector controller 321 has to control the third-order distortion vector regulator 141 until it is determined that the power of the third-order distortion component observed is minimized by the control of the third-order distortion vector regulator 141.
Therefore, there is a need to digitize distortion components at the ADC 31 with a high accuracy, that is, to improve the accuracy of quantization of distortion components, in order to optimize (improve the accuracy of) distortion compensation by the digital predistorter.